As a method replacing a conventional radiography, there is known a radiation image recording and reproducing method utilizing stimulable phosphor, as described in JP-A 55-12145 (herein, the term "JP-A" means an unexamined and published Japanese Patent Application). In the method, a radiation image conversion panel (in other words, an image storage phosphor sheet) comprising a stimulable phosphor is employed, and the method comprises the steps of causing the stimulable phosphor of the panel to absorb radiation having passed through an object or having radiated from an object, sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission), photoelectrically detecting the emitted light to obtain electric signals, and reproducing the radiation image of the object as a visible image from the electric signals. The panel having been read out is subjected to image-erasing and prepared for the next photographing cycle. Thus, the radiation image conversion panel can be used repeatedly.
The stimulable phosphor, after being exposed to radiation, exhibits stimulated emission upon exposure to the stimulating ray. In practical use, phosphors are employed, which exhibit an emission within a wavelength region of 300 to 500 nm stimulated by stimulating light with wavelengths of 400 to 900 nm.
The radiation image conversion panel employed in the radiation image recording and reproducing method basically comprises a support and provided thereon a phosphor layer (stimulable phosphor layer), provided that, in cases where the phosphor layer is self-supporting, the support is not necessarily required. The stimulable phosphor layer comprises a stimulable phosphor dispersed in a binder. There is also known a stimulable phosphor layer, which is formed by vacuum evaporation or a sintering process, free from a binder and comprises an aggregated stimulable phosphor.
There is further known a radiation image conversion panel in which a polymeric material is contained in the openings among the aggregated stimulable phosphor. In these phosphor layers, the stimulable phosphor also exhibits stimulated emission upon exposure to the stimulating rays after absorbing radiation such as X-rays, so that the radiation having passed through an object or having been emitted from the object, is absorbed by the stimulable phosphor layer of the radiation image conversion panel, in proportion to the radiation amount and a radiation image of the object is formed on the panel, as a storage image of radiation energy. The storage image can be released by irradiating the stimulating ray, as stimulating emission light, which is photoelectrically read and transformed into electric signals to form an image as the storage image of radiation energy.
On the surface of the stimulable phosphor layer (i.e., the surface which is not in contact with the support) is conventionally provided a protective layer comprising a polymeric film or an evaporated inorganic membrane to protect the phosphor layer from chemical deterioration and physical shock.
Examples of the stimulable phosphor used in the radiation image conversion panel include,
(1) a rare earth activated alkaline earth metal fluorohalide phosphor represented by the formula of (Ba.sub.1-x, M.sup.2+.sub.X)FX:yA, as described in JP-A 55-12145, in which M.sup.2+ is at least one of Mg, Ca, Sr, Zn and Cd; X is at least one of Cl, Br and I; A is at least one of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb, and Er; x and y are numbers meeting the conditions of 0.ltoreq.x.ltoreq.0.6 and 0.ltoreq.y.ltoreq.0.2; and the phosphor may contain the following additives: PA1 (2) a divalent europium activated alkaline earth metal halide phosphor described in JP-A 60-84381, represented by the formula of M.sup.2 X.sub.2 .multidot.aM.sup.2 '.sub.2 :xEu.sup.2+ (in which M.sup.2 is an alkaline earth metal selected from the group of Ba, Sr and Ca; X and X' is a halogen atom selected from the group of Cl, Br and I and X.noteq.X'; a and x are respectively numbers meeting the requirements of 0.ltoreq.a.ltoreq.0.1 and 0.ltoreq.x.ltoreq.0.2); PA1 the phosphor may contain the following additives; PA1 (3) a rare earth element activated rare earth oxyhalide phosphor represented by the formula of LnOX:xA, as described in JP-A 55-12144 (in which Ln is at least one of La, Y, Gd and Lu; A is at least one of Ce and Tb; and x is a number meeting the following condition, 0&lt;x&lt;0.1); PA1 (4) a cerium activated trivalent metal oxyhalide phosphor represented by the formula of M.sup.3 OX:xCe, as described in JP-A 58-69281 (in which M.sup.3 is an oxidized metal selected from the group of Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Bi; X is a halogen atom selected from the group of Cl, Br and I; x is a number meeting the following condition, 0&lt;x&lt;0.1; PA1 (5) a bismuth activated alkali metal halide phosphor represented by the formula of M.sup.1 X:xBi, as described in Japanese Patent Application No.60-70484 (in which M.sup.1 is an alkali metal selected from the group of Rb and Cs; X is a halogen atom selected from the group of Cl, Br and I; x is a number meeting the following condition, 0&lt;x.ltoreq.0.2; PA1 (6) a divalent europium activated alkaline earth metal halophosphate phosphor represented by the formula of M.sup.2.sub.5 (PO.sub.4).sub.3 X:xEu.sup.2+, as described in JP-A 60-141783 (in which M.sup.2 is an alkaline earth metal selected from the group of Ca, Sr and Ba; X is a halogen atom selected from the group of F, Cl, Br and I; x is a number meeting the following condition, 0&lt;x.ltoreq.0.2); PA1 (7) a divalent europium activated alkaline earth metal haloborate phosphor represented by the formula of M.sup.2.sub.2 BO.sub.3 X:xEu.sup.2+, as described in JP-A 60 157099 (in which M.sup.2 is an alkaline earth metal selected from the group of Ca, Sr and Ba; X is a halogen atom selected from the group of Cl, Br and I; x is a number meeting the following condition, 0&lt;x.ltoreq.0.2); PA1 (8) a divalent europium activated alkaline earth metal halophosphate phosphor represented by the formula of M.sup.2.sub.2 PO.sub.4 X:xEu.sup.2+, as described in JP-A 60-157100 (in which M.sup.2 is an alkaline earth metal selected from the group of Ca, Sr and Ba; X is a halogen atom selected from the group of Cl, Br and I; x is a number meeting the following condition, 0&lt;x.ltoreq.0.2); PA1 (9) a divalent europium activated alkaline earth metal hydrogenated halide phosphor represented by the formula of M.sup.2 HX:xEu.sup.2+, as described in JP-A 60-217354 (in which M.sup.2 is an alkaline earth metal selected from the group of Ca, Sr and Ba; X is a halogen atom selected from the group of Cl, Br and I; x is a number meeting the following condition, 0&lt;x.ltoreq.0.2); PA1 (10) a cerium activated rare earth complex halide phosphor represented by the formula of LnX.sub.3 .multidot.aLn'X.sub.3 ':xCe.sup.3+, as described in JP-A 61-21173 (in which Ln and Ln' are respectively a rare earth element selected from the group of Y, La, Gd and Lu; X and X' are respectively a halogen atom selected from the group of F, Cl, Br and I and X.noteq.X'; a and x are respectively numbers meeting the following conditions, 0.1&lt;a.ltoreq.10.0 and 0&lt;x.ltoreq.0.2; PA1 (11) a cerium activated rare earth complex halide phosphor represented by the formula of LnX.sub.3 .multidot.aM.sup.1 X'X:xCe.sup.3+, as described in JP-A 61-21182 (in which Ln and Ln' are respectively a rare earth element selected from the group of Y, La, Gd and Lu; M.sup.1 is an alkali metal selected from the group of Li, Na, k, Cs and Rb; X and X' are respectively a halogen atom selected from the group of Cl, Br and I; a and x are respectively numbers meeting the following conditions, 0.1&lt;a.ltoreq.10.0 and 0&lt;x.ltoreq.0.2; PA1 (12) a cerium activated rare earth halophosphate phosphor represented by the formula of LnPO.sub.4 .multidot.aLnX.sub.3 :xCe.sup.3+, as described in JP-A 61-40390 (in which Ln is a rare earth element selected from the group of Y, La, Gd and Lu; X is a halogen atom selected from the group of F, Cl, Br and I; a and x are respectively numbers meeting the following conditions, 0.1&lt;a.ltoreq.10.0 and 0&lt;x.ltoreq.0.2; PA1 (13) a divalent europium activated cesium rubidium halide phosphor represented by the formula of CsX:aRbX':xEu.sup.2+, as described in Japanese Patent Application No.60-78151 (in which X and X' are respectively a halogen atom selected from the group of Cl, Br and I; a and x are respectively numbers meeting the following conditions, 0.1&lt;a.ltoreq.10.0 and 0&lt;x.ltoreq.0.2; PA1 (14) a divalent europium activated complex halide phosphor represented by the formula of M.sup.2 X.sub.2 .multidot.aM.sup.1 X':xEu.sup.2+, as described in Japanese Patent Application No.60-78153 (in which M.sup.2 is an alkaline earth metal selected from the group of Ba, Sr and Ca; M.sup.1 is an alkali metal selected from the group of Li, Rb and Cs; X and X' are respectively a halogen atom selected from the group of Cl, Br and I; a and x are respectively numbers meeting the following conditions, 0.1&lt;a&lt;20.0 and 0&lt;x&lt;0.2. PA1 1) there is too large a difference in temperature between the surface and the interior of the powder bed, placed on the boat, during calcination; PA1 2) it is hard for an atmosphere of treatment gas such as reducing gas to reach the interior of the powder bed; PA1 3) since sintering occurs with calcination at a high temperature, it is hard to achieve completion of calcination and reduction in the interior of, or between, powder particles; and PA1 4) in the case of BaFX, heating for a long time to obtain activation results in the release of an amount of halogen gas from the raw material and the gas deposits, without diffusing, in the interor of the powder bed, leading to discoloring and deterioration of desired characteristics. PA1 (1) a radiation image conversion panel comprising a support having thereon a phosphor layer containing a binder and a stimulable phosphor, wherein, when the phosphor layer is excited by the light at a maximum excitation wavelength (.lambda.1) of the radiation image conversion panel, a maximum peak intensity of instantaneous emission (Int.), emitted from the phosphor layer, is within the following range: PA1 provided that said Int. is a maximum emission intensity at a wavelength of 185 to 800 nm, except for wavelengths of .lambda.1 and 2.times. .lambda.1, and said Int. being expressed as a relative value, based on a peak intensity of Raman scattering of water measured with light at a wavelength of 350 nm being 1; PA1 (2) the radiation image conversion panel described in (1), wherein Int. is preferably within the following range: PA1 (3) the radiation image conversion panel described in (1) PA1 wherein the preferred stimulable phosphor is represented by the following formula (1): EQU (Ba.sub.1-x M.sup.2.sub.x)FX:yEu.sup.2+ formula ( 1) PA1 wherein M.sup.2 is an alkaline earth metal selected from the group consisting of Mg, Ca, Sr, Zn and Cd; X is a halogen atom selected from the group consisting of Cl, Br and I; and x and y are numbers within the range of 0.ltoreq.x.ltoreq.0.6 and 0&lt;y.ltoreq.0.2, respectively; PA1 (4) the radiation image conversion panel described in (3), wherein X is preferably I; PA1 (5) the radiation image conversion panel described in (1), wherein the stimulable phosphor is preferably comprised of phosphor particles with an average particle size of 0.1 to 3 .mu.m; PA1 (6) the radiation image conversion panel described in (1), wherein at least 10% by weight of the stimulable phosphor is preferably particles with sizes of 0.1 to 1.0 .mu.m; PA1 (7) the radiation image conversion panel described in (1), wherein a packing ratio of the stimulable phosphor contained in the phosphor layer is preferably 60 to 80%. PA1 500.ltoreq.Int..ltoreq.5000 provided that Int. is the maximum emission intensity at a wavelength within the range of 185 to 800 nm, except for wavelengths of .lambda.1 and 2.times. .lambda.1. The maximum excitation wavelength can be determined using a fluorophotometer, as follows. The phosphor layer is excited by being exposed to a monochromatic light, which is spectrally separated through a monochrometer, and the of intensity of instantaneous emission intensity, emitted from the phosphor layer, is measure with the fluorophotometer, at varying wavelengths of the light, wherein the wavelength at which the maximum emission is obtained is the maximum excitation wavelength, .lambda.1. Herein the intensity, Int., is shown as a relative value, based on the peak intensity of Raman scattering of water measured with light of a wavelength of 350 nm being 1. In cases where the radiation image conversion panel is provided with a protective layer, the protective layer is removed so that phosphor layer becomes the outermost layer of the panel. Further, in cases where at least one of the constituting layers is colored, the colored layer must be decolorized before subjecting to the intensity measurement. PA1 (a) preparing an aqueous mother liquor containing at least 2N BaX.sub.2 (preferably at least 2.7N BaX.sub.2), and a halide of Eu, provided that when x of formula (1) is not zero, the mother liquor further contains a halide of M.sup.2, PA1 (b) adding an aqueous solution containing an at least 5N inorganic fluoride (preferably, ammonium fluoride or alkali metal fluoride) to the mother liquor while maintaining a temperature of the mother liquor at 50.degree. C. or more (preferably 80.degree. C. or more) to form a crystalline precipitate of a precursor of a rare earth activated alkaline earth metal fluorohalide stimulable phosphor, PA1 (c) separating the precipitate of the precursor from mother liquor, and PA1 (d) calcining the separated precipitate (preferably, performing calcination of the precipitate while avoiding sintering of the precipitate). PA1 (a) preparing an aqueous mother liquor containing an at least 3N ammonium halide (preferably, at least 4N), and a halide of Eu (i.e., a chloride, bromide or iodide thereof), provided that when x of formula (1) is not zero, the mother liquor further contains a halide of M.sup.2, when y is not zero, the mother liquor further contains an alkoxide compound of M.sup.2 and when Z is not zero, the mother liquor further contains a halide of M.sup.3, PA1 (b) adding an aqueous solution containing an at least 5N (preferably, at least 8N) inorganic fluoride (ammonium fluoride or alkali metal fluoride) and an aqueous solution containing BaX.sub.2 to the mother liquor while maintaining a temperature of the mother liquor at 50.degree. C. or more (preferably, adding the solutions with keeping constant a ratio of fluorine of the former solution to barium of the latter solution) to form a crystalline precipitate of a precursor of a rare earth activated alkaline earth metal fluorohalide stimulable phosphor, PA1 (c) separating the precipitate of the precursor from the mother liquor, and PA1 (d) calcining the separated precipitate (preferably, performing calcination of the precipitate while avoiding sintering of the precipitate). PA1 1) moving the center of gravity of the powder sample in a dish or a boat within an electric furnace and providing it with a stirrer or a shaker; PA1 2) moving the center of gravity of the powder sample in a dish or a boat by blowing a gas onto or into the sample; and PA1 3) using a rotary electric furnace (rotary kiln), for example, a reaction vessel with rotary or semi-rotary reciprocation within an electric furnace to stir and mix the powder. PA1 rotation speed; 1-50 rpm, preferably 1-20 rpm, PA1 reaction vessel; quartz or SUS (stainless steal), and a rotary blade for stirring may be provided therein. Beads (quartz or ceramics) or balls with a diameter of 5 to 30 mm may concurrently be present therein for the purpose of stirring and mixing, and thereby excessive sintering can be prevented. The reason for providing vibration or fluidization of the powder during calcination is that if powder particles are stirred during calcination, heat and atmospheric gas such as a reducing gas can sufficiently and uniformly reach the interior of the particles, leading to complete calcination within a short time. It is also advantageously effective for the structure of the phosphor, elimination of an element and prevention of excessive sintering.
X', BeX" and M.sup.3 X.sub.3 '", as described in JP-A 56-74175 (in which X', X" and X'", are respectively a halogen atom selected from the group of CL, Br and I; and M.sup.3 is a trivalent metal); PA2 a metal oxide described in JP-A 55-160078, such as BeO, BgO, CaO, SrO, BaO, ZnO, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, La.sub.2 O.sub.3, In.sub.2 O.sub.3, SiO.sub.2, TiO.sub.2, ZrO.sub.2, GeO.sub.2, Sn0.sub.2, Nb.sub.2 O.sub.5 or ThO.sub.2 ; PA2 Zr and Sc described in JP-A 56-116777; PA2 B described in JP-A 57-23673; As and Si described in JP-A 57-23675; PA2 M.multidot.L (in which M is an alkali metal selected from the group of Li, Na, K, Rb and Cs; L is a trivalent metal Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga, In, and Tl) described in JP-A 58-206678; PA2 calcined tetrafluoroboric acid compound described in JP-A 59-27980; PA2 calcined, univalent or divalent metal salt of hexafluorosilic acid, hexafluorotitanic acid or hexafluorozirconic acid described in JP-A 59-27289; PA2 NaX' described in JP-A 59-56479 (in which X' is at least one of Cl, Br and I); PA2 a transition metal such as V, Cr, Mn, Fe, Co or Ni, as described in JP-A 59-56479; PA2 M.sup.1 X', M'.sup.2 X", M.sup.3 X'" and A, as described in JP-A 59-75200 (in which M.sup.1 is an alkali metal selected from the group of Li, Na, K, Rb and Cs; M'.sup.2 is a divalent metal selected from the group of Be and Mg; M.sup.3 is a trivalent metal selected from the group Al, Ga, In and Tl; A is a metal oxide; X', X" and X'" are respectively a halogen atom selected from the group of F, Cl, Br and I);M.sup.1 X' described in JP-A 60-101173 (in which M.sup.1 is an alkali metal selected from the group of Rb and Cs; and X' is a halogen atom selected from the group of F, Cl, Br and I); PA2 M.sup.2 'X'.sub.2 .multidot.M.sup.2 'X".sub.2 (in which M.sup.2 ' is at least an alkaline earth metal selected from the group Ba, Sr and Ca; X' and X" are respectively a halogen atom selected from the group of Cl, Br and I, and X'.noteq.X"); and PA2 LnX".sub.3 described in Japanese Patent Application No. 60-106752 (in which Ln is a rare earth selected from the group of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; X" is a halogen atom selected from the group of F, Cl, Br and I); PA2 M.sup.1 X" described in JP-A 60-166379 (in which M.sup.1 is an alkali metal selected from the group of Rb, and Cs; X" is a halogen atom selected from the group of F, Cl, Br and I; PA2 KX", MgX.sub.2 '" and M.sup.3 X.sub.3 "" described in JP-A 221483 (in which M.sup.3 is a trivalent metal selected from the group of Sc, Y, La Gd and Lu; X", X'" and X"" are respectively a halogen atom selected from the group of F, Cl Br and I; PA2 B described in JP-A 60-228592; PA2 an oxide such as SiO.sub.2 or P.sub.2 O.sub.5 described in JP-A 60-228593; PA2 LiX" and NaX" (in which X" is a halogen atom selected from the group of F, Cl, Br and I; PA2 SiO described in JP-A 61-120883; PA2 SnX.sub.2 " described in JP-A 61-120885 (in which X" is a halogen atom selected from the group of F, Cl, Br and I; PA2 CsX" and SnX.sub.2 '" described in JP-A 61-235486 (in which X" and X'" are respectively a halogen atom selected from the group of F, Cl, Br and I; PA2 CsX" and Ln.sup.3+ described in JP-A 61-235487 (in which X" is a halogen atom selected from the group of F, Cl, Br and I; Ln is a rare earth element selected from the group of Sc, Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; PA2 500.ltoreq.Int..ltoreq.5000 PA2 900.ltoreq.Int..ltoreq.2500;
To obtain enhanced stimulated emission of a radiation image conversion panel, it is indispensable to activate an activator contained in the phosphor in preparation of conventionally known stimulable phosphors. For example, a rare earth metal such as Eu or Ce is contained, as an activator, in a BaFBr phosphor in an amount of 1.times.10.sup.-2 to 1.times.10.sup.-8 mol, and after being subjected to calcination treatment, the activator is occluded in the BaFBr crystal body in the form of Eu.sup.2+ or Ce.sup.2+, leading to enhanced stimulated emission.
In conventional calcination methods, raw phosphor materials are weighed and mixed; after being subjected to calcination in a fixed bed type electric furnace, are than pulverized using a mortar, and subsequently employed as a phosphor to form a phosphor. Herein, "fixed bed type" means that a powder sample is put in a vessel or a boat-shaped dish and calcined as such. A calcination apparatus employed may be an electric muffle furnace or ring furnace. Further, either before or after calcination, the phosphor is optionally subjected to wet mixing, washing, drying or sieving. In some cases, after being calcined and pulverized, the phosphor is further subjected to calcination to enhance calcination efficiency. The conventional calcination method is described in JP-B 1-26640, 63-55555 and 63-28955 (herein, the term "JP-B" means an examined and published Japanese Patent).
Although the emission mechanism of the stimulable phosphor is not sufficiently elucidated, in the case of BaFBr, it is assumed to be necessary that Ba is substituted by an activator such as Eu in its crystal structure and that Eu is reduced from Eu.sup.3+ to Eu.sup.2+. The substitution and reduction can be achieved by calcination at a high temperature, in an atmosphere of reducing gas. The extent of the activating treatment at a high temperature can be deduced by measuring the emission intensity of a phosphor.
Stimulable phosphors according to the prior art are low in sensitivity due to insufficient activation of the phosphor via calcination. Particularly in a BaFX phosphor, with formation of an activator,the amount of Eu.sup.2+ was insufficient. To enhance the absorbing amount of X-rays in a radiation image conversion panel, it was attempted to substitute a halogen element with another element having a higher atomic number. For example, it was attempted to prepare phosphors such as BaF(Br.sub.x,I.sub.1-x):Eu.sup.2+ and BaFI:Eu.sup.2+ by substituting a part of X with I (iodine), but formation of Eu.sup.2+ was insufficient, leading to unacceptably low sensitivity.
According to the inventors of the present invention, the reasons for insufficient calcination, according to the prior art, are as follows:
In general, the smaller the stimulable phosphor particles, the more advantageous they are for graininess and sharpness, and thereby the filling ratio of the phosphor in a phosphor layer can be increased. An increase of the filling ratio of the phosphor in a phosphor layer leads to improved characteristics, such as enhancement of emission intensity. The phosphor described above has an average particle size of 3 to 7 .mu.m. To increase the filling ratio, the content of the binder is decreased and the phosphor layer is subjected to compression treatment to reduce voids. However, when phosphor particles at a size of 3 .mu.m or more are subjected to compression, the phosphor particles are destroyed to form planar particles, leading to deterioration of sharpness of the radiation image conversion panel. Therefore it is desirable to form the phosphor layer without applying compression. It is effective to increase the filling ratio by concurrently incorporating phosphor particles of a smaller size. However, it has been proved that the use of smaller phosphor particles produced problems such that instantaneous emission emitted upon exposure to X rays increased and stimulated emission decreased so that afterglow of instantaneous emission became signal noise when reading the radiation image.